1. Field of the Invention
This invention relates to a rotary kiln adapted for the treatment, such as drying or calcination, of particulate materials by direct heat.
2. Prior Art
Generally, directly heated rotary kilns are commonly employed to dry and/or calcine solid particles such as sand, gravel, limestone, dolomite, magnesite, fertilizers, various metal oxides, and the like. Typically, such kilns are in the form of an elongated rotating cylinder, inclined slightly from the horizontal. The particulate solids to be treated, enter at one end of the kiln and, under the influence of gravity, move toward the other end where the treated product is discharged. Such kilns frequently utilize a method of heat transfer in which solids, especially particulate solids, are heated by direct contact with hot gases. In most direct heat rotary dryers or kilns, heated gases are passed through, either in the same direction as that of the movement of the particulate solids or in a counterflow direction. Typically, the kiln is slowly rotated about its axis and is tilted or inclined slightly from horizontal to effect a tumbling of the solids and a general forward motion while hot gases, e.g. combustion gases are passed from one end of the kiln to the other and caused to flow over the moving solids. As the drum rotates, the bed of particles is carried or dragged upwardly by friction along the inner surface of the drum until the weight of the particles and the steepness of the slope of particle bed overcomes friction and the particles begin to slide or tumble downwardly to the bottom of the particle bed. This tumbling action continues and the particle bed moves slowly forward as the drum continues to rotate. In such a process mixing is relatively poor, with the result that particles of varying sizes are not equally contacted or exposed to the gases. The efficiency of heat transfer from gas to particulate solids is relatively low.
It is known to improve the efficiency through the use of lifting flights attached to the interior wall of the rotating drum. As the drum rotates the lifting flights serve to lift the particles from the bed and then allow them to fall as a shower through the stream of gases as it passes through the kiln. Although thermal efficiency is improved, for some materials, for example, titanium dioxide, the repeated lifting and falling may result in the production of large amounts of fines and dust which may become entrained in the gas stream, resulting in a loss of material and a potential environmental hazard as the dust-laden gases are passed to the atmosphere.
It is also known to pass the hot gases into a direct heat rotary dryer, through the particle bed, in a counter-flow direction, using a Roto-Louvre dryer, wherein hot gases are blown through louvers in a double-wall rotary cylinder and up through the moving bed of particulate solids.
U.S. Pat. No. 1,185,899 discloses a hot air dryer comprising a rotatable cylinder extending through a firebox and means for passing heated air from the firebox lengthwise through the cylinder. The passage of the material to be dried, through the cylinder, is aided by a plurality of flights disposed within the cylinder.
U.S. Pat. No. 3,799,735 discloses the treatment of particulate material with counterflow of combustion gases in a rotary kiln wherein the kiln includes trough shaped conveyor flights. As the particles move along the kiln, they are alternately lifted and dropped to form clouds of parallel vertical curtains through which the combustion gases are passed.
U.S. Pat. No. 4,535,550 discloses a method and apparatus for processing particulate material by passing the material through ah inclined rotary cylinder while the material is contacted with a stream of gas. The gas is introduced into the particle bed through a series of internal supply pipes.
U.S. Pat. No. 5,312,599 discloses a rotary furnace apparatus for the manufacture of activated carbon wherein a rotating bed of particulate material is heated by indirect heating while a controlled flow of sweep gas contacts the surface of the moving particle bed. The sweep gas is introduced through a series of orifices in a sparger tube extending along the length of and within the furnace structure. The gas sweeps across the surface of the particle bed in a direction not parallel to the direction of movement of the bed and exits through a series of inlet ports in an opposing exhaust tube.